The present invention relates to a silicone rubber composition which on heat-curing can afford a rubbery elastomer superior in high voltage electrical insulating characteristics.
High voltage electrical insulators such as those (e.g. suspension insulator and LP insulator) for transmission lines, etc. and those used in insulating devices (e.g. a bird guard device and a lightning arrester device) for transmission lines, etc. are generally made of porcelain or glass. Under ordinary working conditions, these high voltage electrical insulators stand long use. However, when they are used under a severe contaminative environment, for example, when they are used in a district extending along the seaside or in an industrial district, a leakage current of a high level may be generated by a high electrical stress, or there may occur a dry band discharge which leads to flashover.
In view of such problems there have been proposed various methods for remedying the drawbacks of high voltage electrical insulators made of glass or porcelain.
For example, in U.S. Pat. No. 3,511,698 there is proposed a weather-resistant high voltage electrical insulator comprising a thermosetting resin and a cured, platinum catalyst-containing organopolysiloxane elastomer having a thickness of 0.2 mm or more.
In Japanese Patent Laid Open No. 198604/1984 it is taught that by applying one-part type room temperature-curable organopolysiloxane composition to an outer surface of an electrical insulator made of glass or porcelain there can be retained a high electrical performance even in the presence of outdoor stress such as humidity, contamination, or ultraviolet rays.
Further, in Japanese Patent Publication No. 35982/1978 and U.S. Pat. No. 3,965,065 it is taught that a silicone rubber composition improved in ordinary electrical insulating properties is obtained by heating an organopolysiloxane-aluminum hydrate mixture which is converted to rubber on heat-curing, to a temperature higher than 100.degree. C. for 30 minutes or more.
However, the high voltage electrical insulator disclosed in U.S. Pat. No. 3,511,698 involves the problem that there occurs a localized erosion phenomenon because the organopolysiloxane elastomer contains a platinum catalyst.
The high voltage electrical insulator disclosed in Japanese Patent Laid Open No. 198604/1984 involves the problem that a long time is required because of a one-part type room temperature-curable silicone composition is cured at room temperature.
In all of the prior art refrences, the high voltage electrical insulating performance of the silicone rubber material itself cannot be said fully satisfactory. Under severe contaminative or weather conditions, the silicone rubber material undergoes a high electrical stress, thus causing such a deterioration phenomenon as tracking phenomenon or erosion phenomenon, resulting in that the high voltage electrical insulating performance thereof is deteriorated markedly.
Having made extensive studies for overcoming the above-mentioned problems, the present inventors accomplished this invention.
It is the object of the present invention to provide a silicone rubber composition for high voltage electrical insulators capable of being converted to a rubbery elastomer superior in high voltage electrical insulating characteristics such as resistance to weather, to erosion, to tracking and to arc under severe contaminative or weather conditions after heat-curing.